The present invention relates generally to remote sensing and control systems and, specifically, to a detection and alarm system employing a plurality of remote sensing units which are directly connected to a central monitoring and control center.
There is presently a growing requirement for providing large buildings with systems which can detect emergency conditions. For example, in large apartment or office buildings, smoke detectors and the like may be located throughout the building with each detector then being connected to a central monitoring console, which is to be manned at all times. While the very first systems of this kind required each individual sensing unit to be directly connected by dedicated wires to the central control and monitoring console, various methods of reducing the number of wires needed to interconnect the units with the central console are now known.
It is important to reduce the number of interconnecting wires not only to reduce material costs, but also to reduce the amount of labor and time involved in installing the fire detection system into the building.
One approach toward reducing the large number of wires needed to connect a multiplicity of sensors is disclosed in U.S. Pat. No. 3,921,168, assigned to the assignee hereof. In that patent a system is shown which can permit a plurality of remote units to be connected in parallel to the monitoring and control center by a plurality of signal carrying wires, a monitoring wire, and a control wire. The number of remote units monitored and controlled may be as many 2.sup.n, where n is the number of signal carrying wires comprising the above-mentioned plurality. While this system afforded a major reduction in the number of interconnection wires necessary in large installations employing many remote sensing units, it may be seen that a relatively large number of signal carrying wires would still be required if, say, five hundred sensors are involved.
Another approach to reducing the number of wires required to connect a plurality of fire detection transponders to a central station is set forth in U.S. Pat. No. 4,067,008, wherein DC pulses are used to interrogate the plurality of sensors, each sensor and its associated transponder employs a counter which counts the interrogation pulses and will respond only after the particular interrogation pulses corresponding to the count assigned to that transponder have been received.
Another approach to decreasing the number of interconnection wires involves transmitting a specific word over a data bus to the sensing unit, in order to determine the status of each of the sensing units. Although this approach appears promising, a relatively large data bus is required by the system. Alternatively, time division multiplex (TDM) systems can be used for interrogating, in the manner generally known to the communications industry, a number of transponders connected to a central monitoring station.
While all of these systems are effective in reducing the number of interconnections required, they attendantly involve complex electronic units to code and decode the digital words and/or to provide time division multiplexing.
Another disadvantage in prior systems has been the inability of the system to cope with a grounded monitoring line. A grounded monitoring line can result from an integrated circuit failure, a shorted output transistor in the transponder, or a short to the building ground. A grounded monitoring line causes all devices to go into alarm and to call the Fire Department. This is an undesirable false alarm condition.